Design and analysis of metamaterial based perfect absorbers

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Abstract

Subwavelength light absorbers have an enormous potential on applications such as
photodetection, optoelectronics, solar cells and sensing. Scaling down the device
dimensions provides artificial and advanced properties. That's why achieving
higher performance devices with smaller sizes is the main trend in semiconductor
technology. Design of an electromagnetic wave absorber has two dominant factors
on the performance and spectral operation region: material selection and design
configuration. Perfect light absorbers require an absorbing layer, such as a metal,
semiconductor or any type of absorbing material, to achieve light confinement.
While conventional metals have been mostly the primary choice in designs, there
are various material types other than them which can have advantageous thermal
properties in fabrication, integration or tunability besides having lossy nature.
Although conventional metals are great absorbing materials due to lossy natures,
they are not durable against erosion and oxidation. In the first work,
we scrutinize unprecedented potential of transition metal carbides (TMCs) and
nitrides (TMNs) as optional materials to conventional metals, for realization of
light perfect absorption in an ultra-broad frequency range encompassing all of the
visible (Vis) and near infrared (NIR) regions. To gain insight on the condition
for light perfect absorption, a systematic modeling approach based on transfer
matrix method (TMM) is firstly utilized. Our modeling findings prove that the
permittivity data of these TMCs and TMNs are closely matched with the ideal
data. Thus, they can have stronger and broader absorption behavior compared
to metals. Besides, these ceramic materials are preferred to metals due to the
fact that they have better thermal properties and higher durability against erosion
and oxidation than metals. This could provide the opportunity for design of
highly e cient light harvesting systems with long-term stability. Two different
configurations which are planar and trapezoidal arrays are employed. Numerical
simulations are conducted to optimize the device optical performance for each
of the proposed carbides and nitrides. Our findings reveal that these ceramic
coatings have the broadest absorption response compared to all lossy and plasmonic
metals. In planar configuration, titanium carbide (TiC) has the largest
absorption bandwidth (BW) where an absorption above 0.9 is retained over a
broad wavelength range of 405 nm-1495 nm. In trapezoid architecture, vanadium
nitride (VN) shows the widest BW covering a range from 300 nm to 2500 nm.
The results of this study can serve as a beacon for the design of future high performance
energy conversion devices including solar vapor generation and thermal
photovoltaics where both optical and thermal requirements can be satisfied.
Majority of existing designs necessitate a lithography-step during the fabrication,
which hinders the repeatability, upscaling and large-scale compatibility of
these designs. In the second work, we designed, fabricated and characterized a
lithography free, double functional single Bismuth (Bi) metal nanostructure for
ultra-broadband absorption in the visible and near-infrared, and narrowband response
with ultra-high refractive-index sensitivity in mid-infrared (MIR) range.
The superior permittivity data of Bi over conventional metals is comprehensively
analyzed and explained using systematic modeling approaches based on TMM and
Bruggeman's effective medium theory (EMT). To achieve a large scale fabrication
of the design in a lithography-free route, oblique-angle deposition approach is used
to obtain densely packed and randomly spaced/oriented Bi nanostructures. It has
been shown that this fabrication technique can provide a bottom-up approach to
control the length and spacing of the design. Our characterization findings reveal
a broadband absorption above 0.8 in Vis and NIR, and a narrowband absorption
centered around 6.54 m. Due to densely packed architecture of the Bi nanostructures
and its extraordinary permittivity response, they can provide strong
field confinement in their ultra-small gaps and this could be utilized for sensing
application. An ultrahigh sensitivity of 2.151 m/refractive-index-unit (RIU) is
acquired for this Bi nanostructured absorber, which is, to the best of our knowledge,
the experimentally attained highest sensitivity so far. The simple and large
scale compatible fabrication route of the design together with extraordinary optical
response of Bi coating, makes this design promising for many optoelectronic
and sensing applications.